1. General.
Conventional processes for the separation and recovery of products in a fluid catalytic cracking unit (also referred to as a "cat cracker," "cat cracking," or "FCC" unit) of a petroleum refmery do not provide for efficient recovery of the increased quantities of light olefinic products that may be desirably produced with advances in cracking catalyst technology and cat cracker design. The present invention is a new process for the improved separation and recovery of products, especially ethylene and propylene, produced in the cracking reaction of a cat cracker. More specifically, the present invention is a new process for separation and recovery of products from a cat cracker such as the Deep Catalytic Cracking unit described in more detail herein that utilizes recent advances in technology for increased ethylene and propylene production.
In the cracking reaction of conventional fluid catalytic cracking units of petroleum refineries, a hydrocarbon feedstock may be catalytically converted into a variety of products commonly known as slurry oil, heavy cycle oil, light cycle oil, naphtha, and various components lighter than naphtha. The term "naphtha" as used herein means a process stream that contains predominantly five carbon and heavier chemical components with an end point of approximately 430.degree. F. The term "naphtha" as used herein may include a debutanized stream of cracked hydrocarbons that may be processed and used, for example, as a gasoline blending stock. Of course, the particular products from any particular cat cracker unit depend on a variety of factors including the design and needs of the petroleum refinery. The products that include naphtha and lighter components are separated into various product streams in the section of the refinery commonly referred to as the "gas plant," "gas concentration plant," "gas recovery unit," or "unsaturated gas plant" of the cat cracker unit. These terms are commonly used to refer to that portion of a cat cracking unit that includes the wet gas compressor and equipment downstream of the compressor. The products from the gas plant vary depending on the particular refmery design but commonly include naphtha (as defined herein), C.sub.4 s (butylenes and butanes), propane, propylene, and a stream (commonly referred to as "fuel gas") that contains ethane and lighter components (C.sub.2 and lighter).
Importantly, conventional cat cracker gas plant processes separate, at the front end of the gas plant, an "ethane and lighter" stream from a "propylene and heavier" stream. Such conventional cat cracker gas plant processes that include a deethanization step at the front end of the gas plant are described in various treatises. See Nelson's Petroleum Refinery Engineering (McGraw Hill 1949, pp. 759-810); Meyer's Handbook of Petroleum Refining Processes (Second edition 1997, pp. 3.1-3.112); Petroleum Refinery Distillation by Watkins, Second Edition. The deethanization step at the front end of the cat cracker gas plant, and the other features of the conventional cat cracker gas plant as described herein, do not provide for efficient separation of the light olefinic components (such as ethylene and propylene) of the cat cracker reaction products.
In the conventional cat cracker gas plant, the ethane (C.sub.2) and lighter stream may be routed to the refinery fuel gas system or further processed to separate ethane and ethylene from the methane and lighter components. The propylene and heavier stream from the front end of a conventional gas plant is typically further processed to separate a stream that is a naphtha (predominantly C.sub.5 +) fraction from the C.sub.4 and lighter fraction, which may also be further processed to separate the C.sub.3 s (propane and propylene) and C.sub.4 s or processed directly in an alkylation unit.
The product yields from a conventional cat cracker unit vary depending on a wide range of design and operating parameters, such as feedstock quality, the amount of regenerated catalyst supplied to the riser reactor per volume or mass unit of feed, the temperature at which the cracking reaction takes place, the residence time of the feed in the riser reactor, and the like. The conventional fluid catalytic cracking unit may process one or more feedstocks. Typical feedstocks may include atmospheric gas oils, heavier feedstocks from vacuum distillation units, and streams from other units such as cokers, visbreakers, hydrotreaters, and hydrocrackers. The design criteria for conventional fluid catalytic cracking units depend on the feedstock quality, the amount of coke formed on the cracking catalyst during the reaction, the level of contaminants in the feedstock (such as metal contaminants that deactivate the cracking catalyst) and the like.
2. Increased Demand for Olefinic Products: Deep Catalytic Cracking
New catalysts and cat cracker designs have been developed recently in an effort to respond to increased demand for olefinic products from the fluid catalytic cracking unit of a refinery. These recent developments for increased olefin production include changes to the zeolite catalysts typically used in cat crackers and changes in plant process equipment design. For example, the "Deep Catalytic Cracking" (or "DCC") process yields increased proportions of olefinic compounds in comparison to conventional fluid catalytic crackers. As used herein, the term Deep Catalytic Cracking means the process described in U.S. Pat. Nos. 4,980,053 and 5,326,465, the disclosures of which are incorporated herein by reference, and a process that utilizes the catalyst disclosed in U.S. Pat. Nos. 5,232,675, 5,358,918, and 5,380,690, the disclosures of which are incorporated herein by reference. Also see the Handbook of Petroleum Refining Process, second edition, (1997) edited by Robert A. Meyers and published by McGraw-Hill, Chapter 3.5 (pp. 3.101-3.112) entitled "Deep Catalytic Cracking, the New Lights Olefin Generator," the disclosures of which are incorporated herein by reference. DCC is a fluidized catalytic process for selectively cracking a variety of feedstocks to light olefins. Unlike a steam cracker, the predominant products from a DCC unit are propylenes and butylenes, the direct result of catalytic cracking rather than free radical thermal reactions. The DCC unit may be operated in two distinct modes: Maximum Propylene (Type I) or Maximum Iso-Olefin (Type II). Each mode of operation employs a unique catalyst as well as reaction conditions. DCC reaction products are light olefins (such as ethylene and propylene), high octane gasoline, light cycle oil, dry gas and coke. A small amount of slurry oil is produced.
DCC Maximum Propylene operation (Type I) employs both riser and bed cracking at severe reactor conditions. DCC Maximum Iso-Olefin (Type II) operation utilizes riser cracking, like a modern FCC unit, at slightly milder conditions than a Type I operation.
Each mode of DCC operation employs a unique catalyst as well as reaction conditions. DCC reaction products are light olefins, high octane gasoline, light cycle oil, dry gas and coke. As used herein, dry gas means a stream of hydrogen and methane. A small amount of slurry oil is produced.
Innovations in the areas of catalyst development, process variable selection and anticoking techniques enables the DCC unit to produce significantly more olefins than a conventional FCC unit. Table 1 compares pilot plant product yields for a Deep Catalytic Cracking Unit and a conventional cat cracker (maximum gasoline mode) and illustrates the increased light olefin production from a DCC Unit.
TABLE 1 Pilot Plant Yields DCC Product Slate and Yield Structure Process DCC Type I DCC Type II FCC Material Balance, wt % C.sub.2 Minus 11.9 5.6 3.5 C.sub.3 & C.sub.4 42.2 34.5 17.6 Naphtha 26.9 39.5 54.9 LCO 6.9 10.3 10.4 DO 6.1 5.8 9.3 Coke 6.0 4.3 4.3 Total 100.0 100.0 100.0 Olefin Yield, wt % Ethylene 6.1 2.3 0.8 Propylene 21.0 14.3 4.9 Butylene 14.3 14.7 8.1 Feed: Waxy Chinese Vacuum Gas Oil Pilot Plant Data
As used in Table 1, "C.sub.2 Minus" refers to ethane and lighter components; "C.sub.3 & C.sub.4 " refer to three and four carbon compounds respectively; "Naphtha" has the meaning previously described herein; "LCO" refers to light cycle oil; "DO" refers to decant oil, also known as slurry oil; and "coke" refers to carbonaceous deposits on catalyst resulting from the cracking reaction. The Olefin Yield section of Table 1 shows weight percentages of specific olefin compounds.
Historically, the light gases such as ethylene and propylene separated in the cat cracker gas plant were of lesser value and concern than the gasoline product. However, because olefins are now a significant product from a cat cracker and an important source of revenue, there is currently a need to increase the production and recovery of these olefinic products, such as propylene and ethylene.
As noted in Table 1, the Deep Catalytic Cracking Unit produces more ethylene and propylene than a conventional cat cracker. Whereas a conventional cat cracker reactor typically produces up to approximately 5 weight percent or slightly higher (based on reactor fresh feed mass rate) propylene, proprietary catalytic cracking units that are specifically designed to produce more propylene (such as the Deep Catalytic Cracking unit) can produce up to approximately 20 weight percent (based on fresh feed mass rate) propylene in the reactor.
Existing catalytic cracking units that were not originally designed and constructed for increased olefin production may be modified or "revamped" to incorporate new technology that increases the production of olefinic compounds such as ethylene and propylene. The addition of catalytic cracking catalyst specifically formulated for increased ethylene and/or propylene yields in the catalytic cracking reaction will result in the need for a new gas plant process that efficiently accommodates the increased light olefin production. In addition to catalyst changes, the process equipment of an existing cat cracker may be revised to provide increased yields of light olefms. For example, changes to the riser/reactor and related equipment may provide increased olefin production.
Propylene recovery (the portion of propylene in the gas plant feed recovered as propylene product) in conventional gas plants ranges from 80-95 percent (with the remainder of the propylene being lost to other streams). Some refiners attempt to reduce the amount of light olefins lost to fuel gas streams with a "stand-alone" recovery train to separate C.sub.2 and C.sub.3 olefins from the lighter gases, such as the C.sub.2 and lighter gases from the deethanization step at the front end of the conventional cat cracker gas plant. However, ethylene and propylene recovery via such a stand alone system dedicated to processing the light gases destined for the fuel gas system results in redundancy of expensive processing equipment. Moreover, in gas plant processes with a front-end deethanizing step accomplished in an absorber tower followed by a stand-alone recovery train for the C.sub.2 and lighter stream, the C.sub.4 content of the propane recovered from the C.sub.2 and lighter stream may fluctuate due to swings in the C.sub.4 content of the absorber overhead.
Therefore, in light of new processes with increased olefin yields, such as the Deep Catalytic Cracking process, and the increased light olefin yields that may be accomplished through changes to existing cat cracker units, there exists a need for a new gas plant process that provides efficient recovery of ethylene and propylene from such processes and units.